Factorize the suggestion logic in a dedicated service

The current implementation of nuxeo-platform-suggestbox put all the suggestion logic in a seam component. Most of this logic could be made reusable to other kind of UI (e.g. WebEngine or Android and iOS client through a Content Automation operation).

Another use case for a the service split would be to make it possible for new addons to register new "Intent / Action Suggester" implementations through a dedicated extension point. This would be especially useful for the nuxeo-platform-semantic-entities addon.